Symplify links installation by using multi variable for loop

[bapt]

Summary

This not only simplify the code, but also add extra validation if someone by mistake create a macro with only one argument:
He will receive

make: "/usr/src/share/mk/bsd.links.mk" line 10: Wrong number of words (1) in .for substitution list with 2 vars
make: Fatal errors encountered -- cannot continue
make: stopped in /usr/src/etc/pam.d

[bapt]

Fix echoing what is happening

[imp]

I don't think fmake allowed the multiple variables, so we used to be stuck with the old code. This is a nice cleanup. We've already cut that cord, so no need to be compatible.

[bapt]

Closed by commit rS280122 (authored by @bapt).